To improve the operational reliability of light transmitting devices without making their construction more complex, is a problem still to be solved in the art of optical pyrometry.
The great number of patents granted in various countries (cf. USSR Authors' Certificates Nos 146,533 Int. Cl..sup.2 G OIK 1/00, 1961, and 271,067 Int. Cl.sup.2 G 01/J 5/02, 1970, U.S. Pat. No. 3,745,834, FRG Pat. No. 2,338,532) indicates that the above problem still exists and that attempts have been made to solve it. Generally, the prior art light transmitting device comprises a light transmitting member made from a light-permeable corrosion-resistant refractory material such as for instance, quartz or synthetic corundum, and a tube wherein said light-transmitting member is disposed. The space between the inner surface of the tube and the light-transmitting member is filled with a refractory powder. When in use the device is mounted in the lining of a metallurgical vessel so that one end (exposed end) of the light-transmitting member is in contact with a melt, and the opposite end extends through the lining outside the vessel and is optically connected to a pyrometer.
Although a great many attempts have been made to improve the operational reliability of a light guide assembly, this has not yet occurred. This, perhaps, can be explained by the fact that in the prior art apparatus, only principal structural members of the light transmitting device were improved without changing the composition of the refractory powder. At the same time it is worth mentioning that the light guide assembly is operated under conditions of thermal shock (sharp changes in the temperature of the exposed end of the light-transmitting member) and high temperature gradients along the exposed end. Under such conditions the role of a refractory powder as protective means is increasing in importance. Therefore, with a decrease in the thickness of the lining and an increase in the temperature of the melt within the metallurgical vessel, stricter requirements are placed upon the protective refractory powder, since in this case the temperature gradient through the depth of the lining increases.
As a refractory powder, for instance, alkali-free heat resistant oxides are used (cf. Austria Pat. No. 280,650 Int. Cl.sup.2 G 01K 1/16, Apr. 27, 1970). In particular zirconium dioxide (ZrO.sub.2) and aluminum oxide (Al.sub.2 O.sub.3) are most extensively used for this purpose. In the apparatus according to this patent, the powder is used in the same form as it is produced, that is, without preliminary treatment.
It is to be noted that the most efficient light-transmitting members at present are those made from synthetic (monocrystalline) corundum. However, in the course of operation of light guide assemblies having a corundum light-transmitting member and filled with a powder of any of the above two oxides, there arise a number of difficulties.
First, when in use, and in particular at the sintering temperature of the lining, the aluminum oxide powder tends to fuse with the corundum of the light-transmitting member, thereby preventing free displacement of the light-transmitting member within the refractory material, and thus increasing the probability of damage to it.
It should be noted in this connection that although the corundum light-transmitting member has the same chemical composition as the refractory powder, the former is a monocrystal while the latter is in the form of a polycrystal. As a result, they have different linear expansion coefficients, with the linear expansion coefficient of the refractory powder being higher than that of the light-transmitting member, which often results in damage to the latter, when heated, which is highly favored by the change of the temperature gradient across the thickness of the lining.
Furthermore, when sintered, the aluminum oxide powder features low plasticity, which also affects the durability of the light-transmitting member.
In addition, when sintered, this powder has a low thermal stability, which during the course of operation leads to cracks in the refractory mass and breaks off. As a result, the melt which is an aggressive medium gets on the lateral surface of the light-transmitting member, and may result in metal run out through the light guide assembly thereby rendering the metallurgical vessel inoperative and creating hazardous conditions for the attending personnel.
The second known refractory powder which is made from the zirconium dioxide also does not ensure reliable operation of a light guide assembly having a corundum light-transmitting member. Having the same disadvantages as the aluminum oxide e.g., low thermal stability and a linear expansion coefficient differing from that of the material of the light-transmitting member, it has a high sintering temperature so that it does not sinter during the melting operation, and in particular at the sintering temperature of the lining of the metallurgical vessel. Therefore, this powder has to be sintered before use, which complicates the manufacture of the light guide assembly as a whole. In addition, the sintered zirconium dioxide, when in use, does not provide tight contact with the lateral surface of the corundum light-transmitting member and the melt may penetrate through flaws and damage the light-transmitting member. Due to these disadvantages the above light guide assembly is not capable of providing a stable and reliable transmission of thermal radiation from the melt to the pyrometer, and hence a measurement accuracy when measuring the temperature of a melt.